This invention generally relates to an improved oxidation catalyst and its use for catalyzing liquid phase oxidation reactions, especially in acidic oxidative environments and in the presence of reactants, intermediates, products, or solvents which solubilize noble metals. In a preferred embodiment, the present invention relates to an improved oxidation catalyst and a process in which the catalyst is used to convert N-(phosphonomethyl)iminodiacetic acid or a salt thereof into N-(phosphonomethyl)glycine or a salt thereof.
N-(phosphonomethyl)glycine (known in the agricultural chemical industry as xe2x80x9cglyphosatexe2x80x9d) is described in Franz, U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a post-emergent herbicide in an aqueous formulation. It is a highly effective and commercially important broad-spectrum herbicide useful in killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants.
Various methods for making N-(phosphonomethyl)glycine are known in the art. Franz (U.S. Pat. No. 3,950,402) teaches that N-(phosphonomethyl)glycine may be prepared by the liquid phase oxidative cleavage of N-(phosphonomethyl)iminodiacetic acid (sometimes referred to as xe2x80x9cPMIDAxe2x80x9d) with oxygen in the presence of a catalyst comprising a noble metal deposited on the surface of an activated carbon support: 
Other by-products also may form, such as formic acid, which is formed by the oxidation of the formaldehyde by-product; and aminomethylphosphonic acid (xe2x80x9cAMPAxe2x80x9d), which is formed by the oxidation of N-(phosphonomethyl)glycine. Even though the Franz method produces an acceptable yield and purity of N-(phosphonomethyl)glycine, high losses of the costly noble metal into the reaction solution (i.e., xe2x80x9cleachingxe2x80x9d) result because under the oxidation conditions of the reaction, some of the noble metal is oxidized into a more soluble form and both PMIDA and N-(phosphonomethyl)glycine act as ligands which solubilize the noble metal.
In U.S. Pat. No. 3,969,398, Hershman teaches that activated carbon alone, without the presence of a noble metal, may be used to effect the oxidative cleavage of PMIDA to form N-(phosphonomethyl)glycine. In U.S. Pat. No. 4,624,937, Chou further teaches that the activity of the carbon catalyst taught by Hershman may be increased by removing the oxides from the surface of the carbon catalyst before using it in the oxidation reaction. See also, U.S. Pat. No. 4,696,772, which provides a separate discussion by Chou regarding increasing the activity of the carbon catalyst by removing oxides from the surface of the carbon catalyst. Although these processes obviously do not suffer from noble metal leaching, they do tend to produce greater concentrations of formaldehyde by-product when used to effect the oxidative cleavage of N-phosphonomethyliminodiacetic acid. This formaldehyde by-product is undesirable because it reacts with N-(phosphonomethyl)glycine to produce unwanted by-products (mainly N-methyl-N-(phosphonomethyl)glycine, sometimes referred to as xe2x80x9cNMGxe2x80x9d) which reduce the N-(phosphonomethyl)glycine yield. In addition, the formaldehyde by-product itself is undesirable because of its potential toxicity. See Smith, U.S. Pat. No. 5,606,107.
Optimally, therefore, it has been suggested that the formaldehyde be simultaneously oxidized to carbon dioxide and water as the PMIDA is oxidized to N-(phosphonomethyl)glycine in a single reactor, thus giving the following reaction: 
As the above teachings suggest, such a process requires the presence of both carbon (which primarily effects the oxidation of PMIDA to form N-(phosphonomethyl)glycine and formaldehyde) and a noble metal (which primarily effects the oxidation of formaldehyde to form carbon dioxide and water). Previous attempts to develop a stable catalyst for such an oxidation process, however, have not been entirely satisfactory.
Like Franz, Ramon et al. (U.S. Pat. No. 5,179,228) teach using a noble metal deposited on the surface of a carbon support. To reduce the problem of leaching (which Ramon et al. report to be as great as 30% noble metal loss per cycle), however, Ramon et al. teach flushing the reaction mixture with nitrogen under pressure after the oxidation reaction is completed to cause re-deposition of the noble metal onto the surface of the carbon support. According to Ramon et al., nitrogen flushing reduces the noble metal loss to less than 1%. Still, the amount of noble metal loss incurred with this method is unacceptable. In addition, re-depositing the noble metal can lead to loss of noble metal surface area which, in turn, decreases the activity of the catalyst.
Using a different approach, Felthouse (U.S. Pat. No. 4,582,650) teaches using two catalysts: (i) an activated carbon to effect the oxidation of PMIDA into N-(phosphonomethyl)glycine, and (ii) a co-catalyst to concurrently effect the oxidation of formaldehyde into carbon dioxide and water. The co-catalyst consists of an aluminosilicate support having a noble metal located within its pores. The pores are sized to exclude N-(phosphonomethyl)glycine and thereby prevent the noble metal of the co-catalyst from being poisoned by N-(phosphonomethyl)glycine. According to Felthouse, use of these two catalysts together allows for the simultaneous oxidation of PMIDA to N-(phosphonomethyl)glycine and of formaldehyde to carbon dioxide and water. This approach, however, suffers from several disadvantages: (1) it is difficult to recover the costly noble metal from the aluminosilicate support for re-use; (2) it is difficult to design the two catalysts so that the rates between them are matched; and (3) the carbon support, which has no noble metal deposited on its surface, tends to deactivate at a rate which can exceed 10% per cycle.
Thus, a need exists for an improved, multi-reaction catalyst and reaction process which oxidizes PMIDA to N- (phosphonomethyl)glycine while simultaneously exhibiting resistance to noble metal leaching and increased oxidation of formaldehyde into carbon dioxide and water (i.e., increased formaldehyde activity).
This invention provides for an improved catalyst for use in catalyzing liquid phase oxidation reactions, especially in an acidic oxidative environment and in the presence of solvents, reactants, intermediates, or products which solubilize noble metals; a process for the preparation of the improved catalyst; a liquid phase oxidation process using such a catalyst wherein the catalyst exhibits improved resistance to noble metal leaching, particularly in acidic oxidative environments and in the presence of solvents, reactants, intermediates, or products which solubilize noble metals; and a liquid phase oxidation process in which PMIDA or a salt thereof is oxidized to form N-(phosphonomethyl)glycine or a salt thereof using such a catalyst wherein the oxidation of the formaldehyde by-product into carbon dioxide and water is increased.
Briefly, therefore, the present invention is directed to a process for oxidizing a reagent in a mixture (typically a solution or a slurry, and most typically a solution), wherein the mixture has the ability to solubilize a noble metal. This process comprises contacting the mixture with an oxidation catalyst in the presence of oxygen. In one embodiment, the catalyst comprises a carbon support having a noble metal at its surface. The catalyst is characterized as yielding no more than about 1.2 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst in a helium atmosphere is heated from about 20 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst comprises a carbon support having a noble metal and a promoter at a surface of the carbon support. In addition, the catalyst is characterized as yielding no more than about 1.2 mmole of carbon monoxide per gram of catalyst when a dry sample of the catalyst, after being heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before being exposed to an oxidant following the heating in the hydrogen atmosphere, is heated in a helium atmosphere from about 20 to about 900xc2x0 C. at a rate of about 10xc2x0 C. per minute, and then at about 900xc2x0 C. for about 30 minutes.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst comprises a carbon support having a noble metal, carbon, and oxygen at a surface of the carbon support. The ratio of carbon atoms to oxygen atoms at the surface is at least about 20:1 as measured by x-ray photoelectron spectroscopy.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst comprises a carbon support having a noble metal, a promoter, carbon, and oxygen at a surface of the carbon support. The catalyst is characterized as having a ratio of carbon atoms to oxygen atoms at the surface which is at least about 20:1 as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst comprises a carbon support having a noble metal at a surface of the carbon support. In addition, the support comprises a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising oxygen and carbon. The ratio of carbon atoms to oxygen atoms in the surface layer is at least about 20:1 as measured by x-ray photoelectron spectroscopy.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst comprises a carbon support having: (a) a noble metal and a promoter at a surface of the carbon support; and (b) a surface layer having a thickness of about 50 xc3x85 as measured inwardly from the surface and comprising carbon and oxygen. The catalyst is characterized as having a ratio of carbon atoms to oxygen atoms in the surface layer of at least about 20:1 as measured by x-ray photoelectron spectroscopy after the catalyst is heated at a temperature of about 500xc2x0 C. for about 1 hour in a hydrogen atmosphere and before the catalyst is exposed to an oxidant following the heating in the hydrogen atmosphere.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst is prepared by a process comprising depositing a noble metal at a surface of a carbon support, and then heating the surface at a temperature of at least about 400xc2x0 C.
In another embodiment directed to the process for oxidizing a reagent in a mixture which can solubilize a noble metal, the catalyst is prepared by a process comprising depositing a noble metal at a surface of a carbon support, and then exposing the surface to a reducing environment. In this embodiment, before the noble metal deposition, the carbon support has carbon and oxygen at the surface of the carbon support in amounts such that the ratio of carbon atoms to oxygen atoms at the surface is at least 20:1 as measured by x-ray photoelectron spectroscopy.
Other features of this invention will be in part apparent and in part pointed out hereinafter.